1. Field of the Invention
The present invention relates to a rear conversion lens which is mounted on the image space side of a camera lens in order to expand the focal length of the object lens. More specifically the present invention is directed to improvements in such rear conversion lens attachable even to very bright objective lenses.
2. Description of the Prior Art
In the art there are known and used various rear conversion lenses adapted for those objective lenses having a large relative aperture. These known rear conversion lenses generally have a magnification in the range of 1.4 to 1.5 and have an advantage that they are attachable to the so-called bright objective lens without darkening the latter so much. The composite lens brightness resulting from the attachment of the rear conversion lens is only about one step lower than that of the original objective lens in terms of step of aperture stop. For this advantage the rear conversion lens has been admitted to be of high utility value in particular when used in combination with a telephoto lens of large relative aperture. Examples of such rear conversion lens having a magnification of around 1.4 and good performance are disclosed in U.S. Pat. No. 4,206,971 (corresponding to Japanese Patent Publication No. 42726/1980) and Japanese Patent Application Laid-Open No. 123515/1983. However, these prior art rear conversion lenses are still unsatisfactory in some respects. For example, in case of the rear conversion lens disclosed in the first-mentioned patent publication, the correction of astigmatism and astigmatic difference is insufficient. The rear conversion lens disclosed in the second-mentioned patent publication has the drawback that when it is attached to a super telephoto lens in which the exit pupil of the objective lens is at a long distance from the image plane, there arises difficulty in obtaining a sufficient quantity of marginal light.